American elm tree named &#39;St. Croix&#39;

ABSTRACT

A new and distinct variety of American elm tree, particularly distinguished by substantial tolerance to an epiphytotic and normally deadly vascular wilt disease of the genus  Ulmus  known as Dutch elm disease.

LATIN NAME

Ulmus americana L.

VARIETAL DENOMINATION

‘St. Croix’

FIELD OF THE INVENTION

This invention relates to a new and distinct variety or cultivar of thedeciduous tree Ulmus americana, commonly known as the American elm.

BACKGROUND OF THE INVENTION

This new variety of American elm was discovered in Afton, Minn., on anagricultural property, which was homesteaded in 1855 (U.S. granted titleto Thomas Persons) before the tree came into existence.

SUMMARY OF THE INVENTION

This new and distinct variety of American elm is typical of the speciesas locally represented in every apparent physical way, with thebotanical description set forth below. This cultivar is, as typical ofthe species, vase-shaped, but in this case open-grown and spreading.Color and canopy density are excellent. The species is represented inUSDA Hardiness zones 2 through 9 but is restricted to some extent byprovenance, meaning seeds from American elm trees growing in Florida arenot likely to prosper in Winnipeg, Manitoba. This cultivar is likely tobe hardy in USDA hardiness zones 2 through 5 or 6. Given that thismature specimen has survived an unabated Dutch elm disease epidemic forover 30 years without visible injury or infection, while young, wild,American elm trees continue to become infected and die all around it,this cultivar is believed to be exceptionally tolerant to Dutch elmdisease.

The tree was asexually reproduced by rooted cuttings and by grafting andbudding onto established wild-type and U. americana ‘Valley Forge’rootstocks. The asexual reproductions ran true to the originallydiscovered tree from root tip to shoot tip and to each other in allrespects.

The asexual reproductions, along with wild-type U. americana, U.americana ‘Miller Park,’ U. americana ‘Valley Forge,’ and U. rubra elmtrees, were inoculated with Ophiostoma novo-ulmi (about 0.5 ml of asolution containing ≧10⁶ spores/ml) by means of a hole about ⅛ inch indiameter drilled into the base of the trees, when 3-4 feet in height.The U. rubra and U. americana ‘Miller Park,’ ‘Valley Forge,’ and ‘St.Croix’ became symptomatic, but survived, whereas the wild-type elmswilted and died (R. A. Blanchette, unpublished data).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing the timber form of ‘St. Croix.’

FIG. 2 is a photograph showing the timber form of ‘St Croix’ in theabsence of leaves.

FIG. 3 is a photograph showing the bark of ‘St. Croix.’

FIG. 4 is a photograph showing the twigs of ‘St. Croix.’

DETAILED BOTANICAL DESCRIPTION OF THE PLANT

The botanical details of this new and distinct variety of American elmtree are as follows:

The ‘St. Croix’ is an exceptionally large specimen of the American elmspecies. The diameter at breast height (dbh; 4.5 feet above the ground)is about 75 inches (19 feet, 8 inches in circumference). It is about 75feet high with a crown spread of about 110 feet. Its age is unknown, butlikely to be between 80 and 110 years.

Vigor as a genetic measure of suitability for the site is excellent.

Hardiness on the USDA hardiness zone map is likely to be in zones 2through 5 or 6.

General health and pest susceptibility: Vitality as a measure of healthis very good, with good canopy density and excellent dark green color.The tree is normally susceptible to extant indigenous pests, all ofminor importance.

Growth habit and rate: The ‘St. Croix’ has an open-grown, spreading,vase-shaped crown. The growth rate is fast and typical of the species inthis part of Minnesota.

Bark is typically divided into grayish, flat-topped ridges, which areseparated by roughly diamond-shaped fissures and which become indefinitein pattern in the canopy. Bark on young branches is smooth withinconspicuous lenticels.

Twigs are slender, zigzag, brown, glabrous or slightly pubescent;lateral buds are about ¼″ long, ovoid, acute but not sharp-pointed,smooth or sparingly downy, chestnut-brown.

Leaves are deciduous, simple, alternate, short-petioled, 2-ranked, darkgreen (closest to 006600 on HTML True Color Chart), 4 to 6 inches long,1 to 3 inches wide and oblong-obovate to elliptical; margin coarselydoubly serrate; apex acuminate; base conspicuously inequilateral;surfaces glabrous (smooth) or slightly scabrous (roughened) above,usually pubescent below; veins alternate, ascending, parallel andextending from central vein to apex of longest serrations.

Flowers are vernal appearing before the leaves unfold, perfect, born inlong-pedicelled fascicles of 3 or 4; ovary flattened, surmounted by adeeply 2-lobed style.

Fruit is a samara maturing in the spring as the leaves unfold; about ½inch long, oval to oblong-obovate, deeply notched at apex, marginciliate with smooth surfaces.

1. A new and distinct variety of American elm tree named ‘St. Croix’substantially as illustrated and described.